The present invention relates generally to immunological carrier systems. More particularly, the invention pertains to leukotoxin-GnRH chimeras including more than one copy of a GnRH polypeptide. The chimeras demonstrate enhanced immunogenicity as compared to the immunogenicity of GnRH polypeptides alone.
In vertebrates, synthesis and release of the two gonadotrophic hormones, luteinizing hormone (LH) and follicle stimulating hormone (FSH), are regulated by a polypeptide referred to as Gonadotropin releasing hormone (GnRH) (formerly designated LHRH). Accordingly, one approach to fertility control in an animal population is to reduce the levels of GnRH, such as by immunization against GnRH, which effects a reduction in the levels of LH and FSH and the concomitant disruption of estrous cycles and spermatogenesis. See e.g., Adams et al., J. Anim. Sci. (1990) 68:2793-2802.
Early studies of the GnRH molecule have shown that it is possible to raise antisera in response to repeated injections of synthetic GnRH peptides (Arimura et al., Endocrinology (1973) 93(5):1092-1103). Further, antibodies to GnRH have been raised in a number of species by chemical conjugation of GnRH to a suitable carrier and administration of the conjugate in an appropriate adjuvant (Carelli et al., Proc. Natl. Acad. Sci. (1982) 79:5392-5395). Recombinant fusion proteins comprising GnRH or GnRH-analogues have also been described for use in peptide vaccines for the immunological castration or inhibition of reproductive function of various domesticated and farm animals (Meloen et al., Vaccine (1994) 12(8):741-746; Hoskinson et al., Aust. J. Biotechnol. (1990) 4:166-170; and International Publication Nos. WO 92/19746, published Nov. 12, 1992; WO 91/02799, published Mar. 7, 1991; WO 90/11298, published Oct. 4, 1990 and WO 86/07383, published Dec. 18, 1986).
However, attempts have fallen short of providing adequate immunological sterilization products due to the poor immunogenicity of GnRH peptides and due to the fact that chemical conjugation protocols are difficult to control, rendering substantially heterogenous and poorly-defined GnRH conjugates. Further, peptide vaccines based on GnRH have met with limited success in providing uniform effects on individual animal subjects even after repeated vaccination. In this regard, prior GnRH constructs have failed to provide a uniformly successful immunological sterilization vaccine product due to the fact that GnRH is a small, xe2x80x9cselfxe2x80x9d molecule that is not normally recognized by a subject""s immune system, rendering the molecule poorly immunogenic and inherently unable to induce a significant immune response against endogenous GnRH.
It is generally recognized that the immunogenicity of viral antigens, small proteins or endogenous substances may be significantly increased by producing immunogenic forms of those molecules comprising multiple copies of selected epitopes. In this regard, constructs based on two or four repeats of peptides 9-21 of herpes simplex virus type 1 glycoprotein D (Ploeg et al., J. Immuno. Methods (1989) 124:211-217), two to six repeats of the antigenic circumsporozoite tetrapeptide NPNA of Plasmodium falciparum (Lowell et al., Science (1988) 240:800-802), two or four copies of the major immunogenic site of VP1 of foot-and-mouth disease virus (Broekhuijsen et al., J. gen. Virol. (1987) 68:3137-3143) and tandem repeats of a GnRH-like polypeptide (Meloen et al., Vaccine (1994) 12(8):741-746), have been shown to be effective in increasing the immunogenicity of those molecules.
Small proteins or endogenous substances may also be conjugated to a suitable carrier in order to elicit a significant immune response in a challenged host. Suitable carriers are generally polypeptides which include antigenic regions of a protein derived from an infectious material such as a viral surface protein, or a carrier peptide sequence. These carriers serve to non-specifically stimulate T helper cell activity and to help direct antigen to antigen presenting cells for processing and presentation of the peptide at the cell surface in association with molecules of the major histocompatibility complex (MHC).
Several carrier systems have been developed for this purpose. For example, small peptide antigens are often coupled to protein carriers such as keyhole limpet haemocyanin (Bittle et al., Nature (1982) 298:30-33), tetanus toxoid (Muller et al., Proc. Natl. Acad. Sci. U.S.A. (1982) 79:569-573), ovalbumin, and sperm whale myoglobin, to produce an immune response. These coupling reactions typically result in the incorporation of several moles of peptide antigen per mole of carrier protein. Although presentation of the peptide antigen in multiple copies generally enhances immunogenicity, carriers may elicit strong immunity not relevant to the peptide antigen and this may inhibit the immune response to the peptide vaccine on secondary immunization (Schutze et al, J. Immun. (1985) 135:2319-2322).
Antigen delivery systems have also been based on particulate carriers. For example, preformed particles have been used as platforms onto which antigens can be coupled and incorporated. Systems based on proteosomes (Lowell et al., Science (1988) 240:800-802), immune stimulatory complexes (Morein et al., Nature (1984) 308:457-460), and viral particles such as HBsAg (Neurath et al., Mol. Immunol. (1989) 26:53-62) and rotavirus inner capsid protein (Redmond et al., Mol. Immunol. (1991) 28:269-278) have been developed.
Carrier systems have also been devised using recombinantly produced chimeric proteins that self assemble into particles. For example, the yeast retrotransposon, Ty, encodes a series of proteins that assemble into virus like particles (Ty-VLPs; Kingsman, S. M., and A. J. Kingsman Vacc. (1988) 6:304-306). Foreign genes have been inserted into the TyA gene and expressed in yeast as a fusion protein. The fusion protein retains the capacity to self assemble into particles of uniform size.
Other chimeric protein particles have been examined such as HBsAg, (Valenzuela et al., Bio/Technol. (1985) 3:323-326; U.S. Pat. No. 4,722,840; Delpeyroux et al., Science (1986) 233:472-475), Hepatitis B core antigen (Clarke et al., Vaccines 88 (Ed. H. Ginsberg, et al., 1988) pp. 127-131), Poliovirus (Burke et al., Nature (1988) 332:81-82), and Tobacco Mosaic Virus (Haynes et al., Bio/Technol. (1986) 4:637-641). However, these carriers are restricted in their usefulness by virtue of the limited size of the active agent which may be inserted into the structural protein without interfering with particle assembly.
Finally, chimeric systems have been devised using a Pasteurella haemolytica leukotoxin (LKT) polypeptide fused to a selected antigen. See, e.g., International Publication Nos. WO 93/08290, published Apr. 29, 1993 and WO 92/03558, published Mar. 5, 1992, as well as U.S. Pat. Nos. 5,238,823 and 5,273,889. Inclusion of a LKT carrier portion in a peptide antigen chimera supplies enhanced immunogenicity to the chimera by providing T-cell epitopes having broad species reactivity, thereby eliciting a T-cell dependent immune response in immunized subjects. In this regard, inducement of adequate T-cell help is essential in the generation of an immune response to the peptide antigen portion of the chimera, particularly where the antigen is an endogenous molecule. However, the use of a leukotoxin polypeptide carrier in combination with multiple epitopes of the GnRH peptide has not heretofore been described.
The present invention is based on the construction of novel gene fusions between the P. haemolytica leukotoxin gene, variants thereof, and one or more nucleotide sequences encoding multiple GnRH polypeptides. These constructs produce chimeric proteins that display surprisingly enhanced immunogenicity when compared to the immunologic reaction elicited by administration of GnRH alone.
Thus in one embodiment, the present invention is directed to a chimeric protein comprising a leukotoxin polypeptide fused to one or more multimers wherein each multimer comprises more than one selected GnRH polypeptide. The leukotoxin portion of the chimera acts to increase the immunogenicity of the GnRH polypeptides. More particularly, the GnRH multimers used herein may correspond to more than one copy of a selected GnRH polypeptide or epitope, or multiple tandem repeats of a selected GnRH polypeptide or epitope. Further, GnRH multimers may be located at the carboxyl and/or amino terminal of the leukotoxin polypeptide, at sites internal to the leukotoxin polypeptide, or any combination of such sites. Each GnRH multimer may also correspond to a molecule of the general formula GnRH-X-GnRH, wherein X is selected from the group consisting of a peptide linkage, an amino acid spacer group and [GnRH]n, where n is greater than or equal to 1, and further wherein xe2x80x9cGnRHxe2x80x9d may comprise any GnRH polypeptide. In one particular embodiment, a chimeric protein comprising a leukotoxin polypeptide fused to two GnRH multimers is provided. In this molecule, the C-terminus of one of the GnRH multimers is fused to the N-terminus of the leukotoxin polypeptide, and the N-terminus of the leukotoxin polypeptide is fused to the N-terminus of the other GnRH multimer.
Also disclosed are vaccine compositions comprising the chimeric proteins with a pharmaceutically acceptable vehicle, as well as methods for presenting one or more selected GnRH multimers to a host subject by the administration of an effective amount of the subject vaccine compositions.
In another embodiment, the invention is directed to DNA constructs encoding the chimeric proteins. The DNA constructs comprise a first nucleotide sequence encoding a leukotoxin polypeptide operably linked to one or more selected nucleotide sequences, each selected nucleotide sequence encoding more than one copy of a GnRH polypeptide or epitope.
In yet another embodiment, the invention is directed to expression cassettes comprised of the above-described DNA constructs operably linked to control sequences that direct the transcription thereof, whereby the constructs can be transcribed and translated in a host cell.
In another embodiment, the invention is directed to host cells transformed with these expression cassettes.
Another embodiment of the invention provides a method of producing a recombinant polypeptide. The method comprises (a) providing a population of host cells described above and (b) culturing the population of cells under conditions whereby the chimeric polypeptide encoded by the expression cassette is expressed.
These and other embodiments of the present invention will readily occur to those of ordinary skill in the art in view of the disclosure herein.